1. Field of the Invention
The present invention relates to a method of producing an aluminum or aluminum alloy heat exchanger, especially an automotive heat exchanger, which is resistant to corrosion particularly at and in the vicinity of the brazed connections or junctions between the heat exchanger core and the inlet and outlet pipes or manifolds.
2. Description of Related Art
Because of their high thermal conductivity and excellent corrosion resistance, copper alloys have long been used for fabricating automotive heat exchangers, such as radiators, condensers and evaporators. In recent years, however, aluminum and aluminum alloys have begun to replace the copper alloys in automotive applications due to their generally lower cost and the potential for significant savings in weight.
Automotive heat exchangers typically comprise an assembly of a refrigerant conduit pipe, or pipes (referred to generally as the heat exchanger core) and interposed cooling fins. Refrigerant is flowed into and out from the heat exchanger core through inlet and outlet pipes or manifolds which, in the case of aluminum heat exchangers, are brazed to the heat exchanger core.
Unfortunately, heat exchangers made of aluminum and aluminum alloys are more susceptible to corrosion. Aluminum heat exchangers are particularly susceptible to electrolytic corrosion caused by water-soluble salts which are widespread in the environment. For example, a heat exchanger used as the condenser of an automotive air conditioning apparatus is generally mounted in the front portion of the engine compartment and will be exposed to extended operation at elevated temperatures while at the same time being exposed to the influence of water-soluble salts. These circumstances cause pits to form on the heat exchanger surface or core. Within a short period of time these pits quickly grow and eventually cause holes or cracks to form in the heat exchanger core leading to the leakage of refrigerant. Of course, in many cases the heat exchanger must then be replaced.
In order to prevent refrigerant leakage caused by such pit formation, the surface of automotive heat exchangers generally are chemically treated or are provided with a physical coating in order to form a protective film. Provided a faultless protective film can be formed and maintained on the surface of the heat exchanger, this treatment provides good protection against corrosion. However, it is virtually impossible to form a faultless protective film on the heat exchanger surface, thus satisfactory corrosion resistance is not attained by this method. Moreover, such films as easily damaged by physical impacts (e.g., from collisions, mishandling, etc.), causing cracks to form in the protective film, thus destroyng its integrity.
In another method designed to solve the corrosion problem, metals such as zinc (Zn) and/or tin (Sn) are incorporated into the fin material, in order to lower its electrical potential relative to that of the flat tube or core. In this way, the fin material is preferentially corroded, thus protecting the flat tube from corrosion. This approach, however, requires the use of special fin materials, thus increasing the expense of the heat exchanger. Moreover, the heat exchanger core is protected only in the vicinity of the cooling fins, and receives essentially no protection in areas removed from the fins. For example, in cases where brazing between a main pipe, that is the heat exchanger core, and a refrigerant inlet pipe or a refrigerant outlet pipe, that is the inlet and outlet manifolds, is affected by use of a solder consisting of an aluminum-silicon alloy, pittings often are formed in the vicinity of the solder, thus causing leakage of refrigerant.
It also is known to protect aluminum heat exchangers by coating the heat exchanger core with a flux containing zinc chloride (ZnCl.sub.2), followed by brazing to diffuse the zinc over the surface of the heat exchanger. However, the flux itself is quite corrosive and if excess flux is left on the core it corrodes the tube wall leading to tube failure. Thus, in order to ensure that any excess flux is removed, the heat exchanger must be thoroughly washed after brazing. This procedure introduces another step in the manufacturing process and also requires expensive pollution abatement facilities to handle the wash water.
On the other hand, in cases where a noncorrosive flux is used for brazing the heat exchanger, while the washing of excess flux and problem with flux-related corrosion can be avoided, the above-described problems relating for example to electrolytic corrosion particularly at and in the vicinity of the brazed connections are still present and must be addressed.